1. Field Of The Invention
The present invention relates generally to the combustion of organic materials and more particularly to improved combustion of low density (less than 30 lbs/cu. ft.), low moisture (less than 15%), high fuel nitrogen (greater than 0.5%), agricultural and forest product wastes. Such fuels present unique combustion problems that make firing in plants designed for coal and other traditional fossil fuels impractical from either cost and/or reliability and availbility standpoints.
2. Description Of The Prior Art
Conventional multiple hearth furnaces include a plurality of hearths superposed in vertically spaced relationhip. The fuel material is introduced in the uppermost hearth and is moved by rabble arms across the hearth floors to eventually fall through drop holes to the hearths below. In this manner, the fuel material travels downwardly in a serpentine path. The product discharged from the lowermost hearth contains ash and possibly significant amounts of unburnt fixed carbon, depending on the sizing and combustion control philosophy utilized.
Conventional control strategies for multiple hearth furnace exist to control combustion phenomena to the extent that certain gross parameters are controlled as dictated by either limitations of the hardware and materials of construction, by laws having jurisdiction over the composition and quantities of exhaust products produced, or by gross combustion strategies. For example, automatic temperature controls limit the hearth temperatures to levels below the failure points of the furnace construction. Staged combustion with afterburner, reducing and oxidizing zones improves combustion efficiency and lowers NOx emissions. Feedback from oxygen and CO analyzers control the combustion air to fuel ratios, resulting in further improved combustion efficiencies.
Conventional practice recognizes the advantage of staged combustion in burning organic materials, and in the case of multiple hearth furnace design, attempts have been made to realize both reducing and oxidizing combustion zones in a single furnace shell with common center shaft and rabble arm drive. Combining both zones in a common shell forces compromises in combustion strategies and results in complications of hardware design.
The transition from reducing to oxidizing zones using a multiple hearth furnace as the combustor has to-date presented a significant isolation problem. Multiple hearth furnace designs to-date have attempted to solve this problem with a combination of mechanical seals, controls and product feed devices that limit gas flow from the oxidizing to the reducing zones. The isolation means attempted have proven to be imperfect to the extent that some transfer of gases between zones can occur under certain operating conditions, resulting in process upsets and decreased combustion rate of efficiency, or inflexibility in zone transition definition, resulting in under-utilization of the furnace built, or disharqe of a waste ash product that contains significant unburnt carbon, representing a loss in overall combustion efficiency.
The isolation means attempted have in some cases been suited to dense materials only, relying on product weight to force open spring or counter-balanced valves. Existing designs of mechanical isolation means often locates the sealing or isolating device in a hot, restricted access area, resulting in limited service life and difficult maintenance, which promotes operation of the system with a failed seal, and the resulting decrease in combustion efficiency.
Exemplary of the prior art which disclose one or more of the concepts noted above are U.S. patents to Lewers (U.S. Pat. No. 1,881,732), von Dreusche (U.S. Pat. No. 4,050,389), Bakker (U.S. Pat. No. 3,491,707), Anderson (U.S. Pat. No. 3,444,687), Evans (U.S. Pat. No. 3,658,482), Hazzard (U.S. Pat. No. 3,780,676), Brewer (U.S. Pat. No. 4,038,032), Raische (U.S. Pat. No. 2,104,526), Martin (U.S. Pat. No. 2,655,883) and Lombana (U.S. Pat. No. 4,182,246).
Lewers discloses the broad concept of separating the oxidation section from the reducing section in a MHF and preventing the gases evolved during oxidation from entering the reducing section. Martin and Raische further teach mechanically conveying the product leaving the drying zone to the oxidation zone, without allowing the oxidation gases to enter the drying zone. None of the prior art patents, however, disclose or suggest the method of burning organic materials using the multiple hearth furnace arrangement of the present invention.